5-Phenoxy Primaquine Analogs and the Tetraoxane Hybrid as Antimalarial Agents.

The rapid emergence of drug resistance to the current antimalarial agents has led to the urgent need for the discovery of new and effective compounds. In this work, a series of 5-phenoxy primaquine analogs with 8-aminoquinoline core (7a-7h) was synthesized and investigated for their antimalarial activity against Plasmodium falciparum. Most analogs showed improved blood antimalarial activity compared to the original primaquine. To further explore a drug hybrid strategy, a conjugate compound between tetraoxane and the representative 5-phenoxy-primaquine analog 7a was synthesized. In our work, the hybrid compound 12 exhibited almost a 30-fold increase in the blood antimalarial activity (IC50 = 0.38 ± 0.11 µM) compared to that of primaquine, with relatively low toxicity against mammalian cells (SI = 45.61). Furthermore, we found that these 5-phenoxy primaquine analogs and the hybrid exhibit significant heme polymerization inhibition, an activity similar to that of chloroquine, which could contribute to their improved antimalarial activity. The 5-phenoxy primaquine analogs and the tetraoxane hybrid could serve as promising candidates for the further development of antimalarial agents.

Tetroxanes as New Agents against Leishmania amazonensis.

Leishmaniasis is a neglected disease, caused by a parasite of Leishmania genus and widespread in the tropical and subtropical areas of the world. Currents drugs are limited due to their toxicity and parasite resistance. Therefore, the discovery of new treatment, more effective and less toxic, is urgent. In this study, we report the synthesis of six gem-dihydroperoxides (2a-2f), with yields ranging from 10 % to 90 %, utilizing a new methodology. The dihydroperoxides were converted into ten tetroxanes (3a-3j), among which six (3b, 3c, 3d, 3g, 3h and 3j) showed activity against intracellular amastigotes of Leishmania amazonensis. The cytotoxicity of all compounds was also evaluated against canine macrophages (DH82), human hepatoma (HepG2) and monkey renal cells (BGM). Most compounds were more active and less toxic than potassium antimonyl tartrate trihydrate, used as positive control. Amongst all tetroxanes, 3b (IC50 =0.64 µm) was the most active, being more selective than positive control in relation to DH82, HepG2 and BGM cells. In summary, the results revealed a hit compound for the development of new drugs to treat leishmaniasis.

Anti-Mycobacterial Peroxides: A New Class of Agents for Development Against Tuberculosis.

BACKGROUND: With few exceptions, existing tuberculosis drugs were developed many years ago and resistance profiles have emerged. This has created a need for new drugs with discrete modes of action. There is evidence that tuberculosis (like other bacteria) is susceptible to oxidative pressure and this has yet to be properly utilised as a therapeutic approach in a manner similar to that which has proven highly successful in malaria therapy. OBJECTIVE: To develop an alternative approach to the incorporation of bacterial siderophores that results in the creation of antitubercular peroxidic leads for subsequent development as novel agents against tuberculosis. METHODS: Eight novel peroxides were prepared and the antitubercular activity (H37Rv) was compared to existing artemisinin derivatives in vitro. The potential for toxicity was evaluated against the L6 rat skeletal myoblast and HeLa cervical cancer lines in vitro. RESULTS: The addition of a pyrimidinyl residue to an artemisinin or, preferably, a tetraoxane peroxidic structure results in antitubercular activity in vitro. The same effect is not observed in the absence of the pyrimidine or with other heteroaromatic substituents. CONCLUSION: The incorporation of a pyrimidinyl residue adjacent to the peroxidic function in an organic peroxide results in anti-tubercular activity in an otherwise inactive peroxidic compound. This will be a useful approach for creating oxidative drugs to target tuberculosis.

Catalyst Development for the Synthesis of Ozonides and Tetraoxanes Under Heterogeneous Conditions: Disclosure of an Unprecedented Class of Fungicides for Agricultural Application.

The catalyst H3+x PMo12-x +6 Mox +5 O40 supported on SiO2 was developed for peroxidation of 1,3- and 1,5-diketones with hydrogen peroxide with the formation of bridged 1,2,4,5-tetraoxanes and bridged 1,2,4-trioxolanes (ozonides) with high yield based on isolated products (up to 86 and 90 %, respectively) under heterogeneous conditions. Synthesis of peroxides under heterogeneous conditions is a rare process and represents a challenge for this field of chemistry, because peroxides tend to decompose on the surface of a catalyst . A new class of antifungal agents for crop protection, that is, cyclic peroxides: bridged 1,2,4,5-tetraoxanes and bridged ozonides, was discovered. Some ozonides and tetraoxanes exhibit a very high antifungal activity and are superior to commercial fungicides, such as Triadimefon and Kresoxim-methyl. It is important to note that none of the fungicides used in agricultural chemistry contains a peroxide fragment.

Synthesis and profiling of benzylmorpholine 1,2,4,5-tetraoxane analogue N205: Towards tetraoxane scaffolds with potential for single dose cure of malaria.

A series of aryl carboxamide and benzylamino dispiro 1,2,4,5-tetraoxane analogues have been designed and synthesized in a short synthetic sequence from readily available starting materials. From this series of endoperoxides, molecules with in vitro IC50s versus Plasmodium falciparum (3D7) as low as 0.84 nM were identified. Based on an assessment of blood stability and in vitro microsomal stability, N205 (10a) was selected for rodent pharmacokinetic and in vivo antimalarial efficacy studies in the mouse Plasmodium berghei and Plasmodium falciparum Pf3D70087/N9 severe combined immunodeficiency (SCID) mouse models. The results indicate that the 4-benzylamino derivatives have excellent profiles with a representative of this series, N205, an excellent starting point for further lead optimization studies.

Synthesis and antimalarial activity of 3'-trifluoromethylated 1,2,4-trioxolanes and 1,2,4,5-tetraoxane based on deoxycholic acid.

A series of new steroidal peroxides - 3'-trifluoromethylated 1,2,4-trioxolanes and 1,2,4,5-tetraoxanes based on deoxycholic acid were prepared via the reactions of the Griesbaum coozonolysis and peroxycondensation, respectively. 1,2,4-Trioxolanes were synthesized by the interaction of methyl O-methyl-3-oximino-12α-acetoxy-deoxycholate with CF3C(O)CH3 or CF3C(O)Ph and O3 as the mixtures of four possible stereoisomers at ratios of 1:2:2:1 and in yields of 50% and 38%, respectively. The major diastereomer of methyl 12α-acetoxy-5ß-cholan-24-oate-3-spiro-5'-(3'-methyl-3'-trifluoromethyl-1',2',4'-trioxolane) was isolated via crystallization of a mixture of stereoisomers from hexane and its (3S,3'R)-configuration was determined using X-ray crystallographic analysis. Peroxycondensation of methyl 3-bishydroperoxy-12α-acetoxy-deoxycholate with CF3C(O)CH3 or acetone led to 1,2,4,5-tetraoxanes in yields of 44% and 37%, respectively. Antimalarial activity of these new steroidal peroxides was evaluated in vitro against the chloroquine-sensitive (CQS) T96 and chloroquine-resistant (CQR) K1 strains of Plasmodium falciparum. Deoxycholic acid 3'-trifluoromethylated 1,2,4,5-tetraoxane demonstrated a good IC50 value against CQR-strain (IC50 (K1) = 7.6 nM) of P. falciparum. Tetraoxane with the acetone subunit demonstrated the best results among all tested peroxides with an IC50 value of 3 nM against the CQ-resistant K1 strain. In general, 1,2,4-trioxolanes of deoxycholic acid are less active than 1,2,4,5-tetraoxanes.

Potential of synthetic endoperoxides against Trichomonas vaginalis in vitro.

Metronidazole is well known for medicine against Trichomonas vaginalis infection, but it has side effects though it is effective, and especially because reports of metronidazole-tolerant species are increasing, the development of new medicine is being required. Here, we noticed the killing effects of endoperoxide compounds, N-89 and N-251 as new antimalarial drug candidates, on T. vaginalis and searched the possibility of development of new medicine. We added each of metronidazole, artemisinin, and two of new endoperoxides (N-89 and N-251) to metronidazole-resistant and -sensitive species and compared its anti-trichomonal efficacy. For metronidazole, IC50 value, 50% of killing concentration for T. vaginalis, was very low for metronidazole-sensitive isolates (11.7 to 22.8µM), but was high for metronidazole-resistant ones (182.9 to 730.4µM). The IC50 values of N-89 and N-251 were 41.0 to 60.0µM, and 82.0 to 300.0µM for metronidazole-sensitive and -resistant isolates, respectively. In conclusion, we found the endoperoxides, N-89 and N-251, have anti-trichomonal effect against metronidazole-resistant T. vaginalis as well as metronidazole-sensitive ones. These results indicate that the anti-trichomonal effects for our endoperoxides are equivalent or better in metronidazole-resistant T. vaginalis in comparison to metronidazole.

Reinvestigating Old Pharmacophores: Are 4-Aminoquinolines and Tetraoxanes Potential Two-Stage Antimalarials?

The syntheses and antiplasmodial activities of various substituted aminoquinolines coupled to an adamantane carrier are described. The compounds exhibited pronounced in vitro and in vivo activity against Plasmodium berghei in the Thompson test. Tethering a fluorine atom to the aminoquinoline C(3) position afforded fluoroaminoquinolines that act as intrahepatocytic parasite inhibitors, with compound 25 having an IC50 = 0.31 µM and reducing the liver load in mice by up to 92% at 80 mg/kg dose. Screening our peroxides as inhibitors of liver stage infection revealed that the tetraoxane pharmacophore itself is also an excellent liver stage P. berghei inhibitor (78: IC50 = 0.33 µM). Up to 91% reduction of the parasite liver load in mice was achieved at 100 mg/kg. Examination of tetraoxane 78 against the transgenic 3D7 strain expressing luciferase under a gametocyte-specific promoter revealed its activity against stage IV-V Plasmodium falciparum gametocytes (IC50 = 1.16 ± 0.37 µM). To the best of our knowledge, compounds 25 and 78 are the first examples of either an 4-aminoquinoline or a tetraoxane liver stage inhibitors.

Tetraoxanes as a new class of efficient herbicides comparable with commercial products.

BACKGROUND: Several 1,2,4,5-tetraoxanes were synthesised, and their herbicidal activity was tested against weeds and compared with the activity of commercial herbicides glyphosate and imazethapyr. RESULTS: The compounds were prepared by reacting carbonyl compounds with hydrogen peroxide under acid catalysis, affording 1,1-dihydroperoxides (36-91%) that were further converted into 1,2,4,5-tetraoxanes (10-52%) under similar reaction conditions. All products were evaluated against Sorghum bicolor and Cucumis sativus at 0.0125-1.0 mM, and several tetraoxanes caused >70% inhibition of the growth of roots and aerial parts. The most active products were evaluated against the weeds Sorghum arundinaceum, Euphorbia heterophylla, Brachiaria brizantha and Bidens pilosa. Some compounds were highly effective (>80% inhibition at 1.0 mM) against the weeds, showing activity comparable with that of glyphosate or imazethapyr. One of the tetraoxanes was selective, being inactive against dicotyledonous species while inhibiting the roots and aerial parts of monocotyledonous species by 92.9-97.5%, which is comparable with the effect of glyphosate. CONCLUSIONS: Tetraoxanes constitute a new class of effective herbicides with great potential for commercial development.

Approach for the preparation of various classes of peroxides based on the reaction of triketones with H2O2: first examples of ozonide rearrangements.

The reaction of ß,δ-triketones with an ethereal solution of H2O2 catalyzed by heteropoly acids in the presence of a polar aprotic co-solvent proceeds via three pathways to form three classes of peroxides: tricyclic monoperoxides, bridged tetraoxanes, and a pair of stereoisomeric ozonides. The reaction is unusual in that produces bridged tetraoxanes and ozonides with one of the three carbonyl groups remaining intact. In the synthesis of bridged tetraoxanes, the peroxide ring is formed by the reaction of hydrogen peroxide with two carbonyl groups at the ß positions. The synthesis of ozonides from ketones and hydrogen peroxide is a unique process in which the ozonide ring is formed with the participation of two carbonyl groups at the δ positions. Rearrangements of ozonides were found for the first time after more than one century of their active investigation. Ozonides are interconverted with each other and rearranged into tricyclic monoperoxides, whereas ozonides and tricyclic monoperoxides are transformed into bridged tetraoxanes. The individual reaction products were isolated by column chromatography and characterized by NMR spectroscopy, mass spectrometry, and elemental analysis. One representative of each class of peroxides was characterized by X-ray diffraction.

Tetraoxane-pyrimidine nitrile hybrids as dual stage antimalarials.

The use of artemisinin or other endoperoxides in combination with other drugs is a strategy to prevent development of resistant strains of Plasmodium parasites. Our previous work demonstrated that hybrid compounds, comprising endoperoxides and vinyl sulfones, were capable of high activity profiles comparable to artemisinin and chloroquine while acting through two distinct mechanisms of action: oxidative stress and falcipain inhibition. In this study, we adapted this approach to a novel class of falcipain inhibitors: peptidomimetic pyrimidine nitriles. Pyrimidine tetraoxane hybrids displayed potent nanomolar activity against three strains of Plasmodium falciparum and falcipain-2, combined with low cytotoxicity. In vivo, a decrease in parasitemia and an increase in survival of mice infected with Plasmodium berghei was observed when compared to control. All tested compounds combined good blood stage activity with significant effects on liver stage parasitemia, a most welcome feature for any new class of antimalarial drug.

Tetraoxanes as antimalarials: harnessing the endoperoxide.

With the emergence of resistance to artemesinin, the need for new antimalarial compounds is pressing. Several research groups have made significant contributions to the exploration of the use of 1,2,4,5-tetraoxanes and 1,2,4- trioxanes as synthetic analogues of artemesinin. This short review highlights the recent developments in this field detailing both biological results and useful synthetic methodology. In addition, the current understanding of the mode of action of this class of compounds has been described.

Synthesis and evaluation of the antimalarial, anticancer, and caspase 3 activities of tetraoxane dimers.

The synthesis of a range of mono spiro and dispiro 1,2,4,5-tetraoxane dimers is described. Selected molecules were examined in in vitro assays to determine their antimalarial and anticancer potential. Our studies reveal that several molecules possess potent nanomolar antimalarial and single digit micromolar antiproliferative IC(50)s versus colon (HT29-AK and leukemia (HL60) cell lines.

Phosphomolybdic and phosphotungstic acids as efficient catalysts for the synthesis of bridged 1,2,4,5-tetraoxanes from ß-diketones and hydrogen peroxide.

Phosphomolybdic acid (PMA) and phosphotungstic acid (PTA) efficiently catalyze the addition of H2O2 to ß-diketones to form bridged 1,2,4,5-tetraoxanes. These reactions are not accompanied by the formation of monocyclic peroxides containing hydroxy and hydroperoxide groups or polymeric peroxides. The use of these catalysts made it possible to obtain bridged tetraoxanes from easily oxidizable benzoylacetone derivatives and α-unsubstituted ß-diketones. The syntheses are scaled up to ten grams. The resulting peroxides can be easily isolated from the reaction mixture by column chromatography. The yield of tetraoxanes depends on the structure of ß-diketone and varies from 12 to 83%. NMR monitoring of two bridged 1,2,4,5-tetraoxanes synthesis was carried out.

Tetraoxanes: synthetic and medicinal chemistry perspective.

The discovery of artemisinin from Chinese medicinal plant, Artemisia annua in 1971, opened a new era in the malarial chemotherapy. This discovery was the beginning of exploring peroxides as potential replacements for the traditional antimalarial drugs such as chloroquine and mefloquine. The structurally simple class of peroxides that emerged from these studies was the 1,2,4,5-tetraoxanes. This study describes the current status of tetraoxane-based antimalarials that show significant promises because of their artemisinin-like activity. Literature from 1999 has been critically reviewed and an attempt has been made to discuss various synthetic methods and structure­activity relationship study among the series of tetraoxane-based compounds.

Synthesis and antimalarial activity of dihydroperoxides and tetraoxanes conjugated with bis(benzyl)acetone derivatives.

Dihydroperoxides and tetraoxanes derived from symmetrically substituted bis(arylmethyl)acetones were synthesized in modest to good yields using several methods. Three of these compounds exhibit an important in vitro antimalarial activity (1.0 µm ≤ IC(50) ≤ 5.0 µm) against blood forms of the human malaria parasite Plasmodium falciparum.

Comparison of the reactivity of antimalarial 1,2,4,5-tetraoxanes with 1,2,4-trioxolanes in the presence of ferrous iron salts, heme, and ferrous iron salts/phosphatidylcholine.

Dispiro-1,2,4,5-tetraoxanes and 1,2,4-trioxolanes represent attractive classes of synthetic antimalarial peroxides due to their structural simplicity, good stability, and impressive antimalarial activity. We investigated the reactivity of a series of potent amide functionalized tetraoxanes with Fe(II)gluconate, FeSO(4), FeSO(4)/TEMPO, FeSO(4)/phosphatidylcholine, and heme to gain knowledge of their potential mechanism of bioactivation and to compare the results with the corresponding 1,2,4-trioxolanes. Spin-trapping experiments demonstrate that Fe(II)-mediated peroxide activation of tetraoxanes produces primary and secondary C-radical intermediates. Reaction of tetraoxanes and trioxolanes with phosphatidylcholine, a predominant unsaturated lipid present in the parasite digestive vacuole membrane, under Fenton reaction conditions showed that both endoperoxides share a common reactivity in terms of phospholipid oxidation that differs with that of artemisinin. Significantly, when tetraoxanes undergo bioactivation in the presence of heme, only the secondary C-centered radical is observed, which smoothly produces regioisomeric drug derived-heme adducts. The ability of these tetraoxanes to alkylate the porphyrin ring was also confirmed with Fe(II)TPP and Mn(II)TPP, and docking studies were performed to rationalize the regioselectivity observed in the alkylation process. The efficient process of heme alkylation and extensive lipid peroxidation observed here may play a role in the mechanism of action of these two important classes of synthetic endoperoxide antimalarial.

The activity of dispiro peroxides against Fasciola hepatica.

Dispiro 1,2,4-trioxanes and 1,2,4,5-tetraoxanes had superior efficacy against Fasciola hepatica than the corresponding ozonides (1,2,4-trioxolanes). For highest efficacy, spiroadamantane and carboxymethyl substructures were required. Three compounds completely cured F. hepatica-infected mice at single oral doses of 50mg/kg and two were partially curative at single doses of 25mg/kg.

Synthesis and in vitro antimalarial activity of tetraoxane-amine/amide conjugates.

A series of tetraoxanes, tetraoxane-amine and tetraoxane-amide conjugates have been synthesized and screened for in vitro antimalarial activity against chloroquine-sensitive and chloroquine-resistant strains of Plasmodium falciparum. Most of the conjugates showed slightly better antimalarial activity than the parent tetraoxanes. Three of the conjugate compounds were potentially active with IC(50) values in the range of 0.38-0.80µM. Cytotoxicity of four selected compounds was also evaluated in a panel of four cancer (SK-MEL, KB, BT-549, SK-OV-3) and two non-cancer (Vero and LLC-PK(11)) cell lines up to a concentration of 25µM and none of the compounds was found toxic to any of the cells.

The structure and antimalarial activity of dispiro-1,2,4,5-tetraoxanes derived from (+)-dihydrocarvone.

An unsaturated dispiro 1,2,4,5-tetraoxane formed by peroxidation of (+)-dihydrocarvone was converted into four structurally diverse derivatives. X-ray crystallographic analysis shows that the structures possess central tetraoxane rings with spiro-2,5-disubstituted cyclohexylidene substituents and 6-membered rings in classical chair conformations. As polarity in the tetraoxane series increased, in vitro potency against Plasmodium falciparum decreased.